1. Field of the Invention
The present invention generally relates to liquid crystal display devices, and more particularly to a liquid crystal display device applying a vertical orientation mode.
The liquid crystal display device has been widely used for various portable information processing apparatuses, especially, for laptop computers and cellular phones, as a display device that is minimized and requires lower power consumption. Recently, a performance of the liquid crystal display has been dramatically improved to be faster. Thus, recent technology of the liquid crystal display achieving a faster response speed and a clearer contrast is realized to replace a CRT display device of a desktop computer or a workstation computer.
However, such a conventional liquid crystal display is needed to improve the response speed and the contrast in order to be especially applied to a flat display device of a desktop computer. Also, it is required to realize a wider angle of visibility to recognize information displayed on the conventional liquid crystal display device as much as a direct view angle to a front of the conventional liquid crystal display device.
2. Description of the Related Art
Conventionally, as a practical liquid crystal device, a TN-type liquid crystal display device that is normally in white mode has been widely used. In the TN-type liquid crystal display device, an orientation direction of liquid crystal molecules is changed depending on an applied voltage signal. A transmission light is controlled to turn ON or OFF by the change of the orientation direction of the liquid crystal molecules.
However, in the TN-type liquid crystal display device, a ratio of contrast is limited because of an operation principle. Thus, it is difficult to realize the wider view angle that is required for the display device of the desktop computer.
On the contrary, the inventor of the present invention has already proposed a liquid crystal display device in which the liquid crystal molecules in a liquid layer are oriented to an approximate vertical direction in advance before a driving voltage is applied, so called a vertical orientation liquid crystal display device.
A principle of a vertical orientation liquid crystal display device 10 that is proposed by the inventor of the present invention, called an MVA type, will now be described. FIG. 1A is a diagram showing a non-driving state in that the driving voltage is not applied to the liquid crystal display device 10 and FIG. 1B is a diagram showing a driving state in that the driving voltage is applied to the liquid crystal display device 10.
Referring to FIG. 1A, a liquid crystal layer 12 is clamped between glass substrates 11A and 11B. The glass substrates 11A and 11B form a liquid panel with the liquid crystal layer 12. Molecule orientation films (not shown) are formed on the glass substrates 11A and 11B. By effects of the molecule orientation film, in the state in which the driving voltage is not applied, the liquid crystal molecules in the liquid crystal layer 12 are oriented in the approximate vertical direction to the liquid crystal layer 12. In the non-driving state in FIG. 1A, because a deflection angle of a deflection plate is not substantially changed, in a case in which a polarizer and an analyzer are provided on a top and a bottom surfaces in a crossed Nicol state, an incident light beam, which transmits through the polarizer and enters the liquid crystal layer 12, is interrupted by the analyzer.
On the other hand, in the driving state in FIG. 1B, the liquid molecules are tilted by influence of the applied electric field. Then, the deflection direction of the deflection plate is changed. As a result, the incident light beam, which transmits through the polarizer and enters the liquid crystal layer 12, transmits through the analyzer.
Moreover, in the liquid crystal display device 10 in FIG. 1A and FIG. 1B, during the transmission from the non-driving state to the driving state, in order to suppress a tilt direction of the liquid crystal molecules and improve the response speed, extended projecting patterns 13A and 13B are alternately formed on the glass substrates 11A and 11B in parallel.
By forming the projection patterns 13A and 13B, the response speed of the liquid crystal display device 10 is improved and simultaneously a plurality of domains are formed in different directions of tilting the liquid crystal molecule in the liquid crystal layer 12. As a result, the view angle of the liquid crystal display device 10 is greatly improved.
FIG. 2 is a diagram showing an orientation state of the liquid crystal molecules in vicinities of the projection patterns 13A and 13B in the driving state in FIG. 1B.
Referring to FIG. 2, the liquid crystal molecules are tilted in the driving state and an angle difference is about 180° between orientation directions of the projections 13A and 13B. That is, the projections 13A and 13B are twisted. In FIG. 2, a polarizer absorbent axis P and an analyzer absorbent axis A are shown.
In liquid crystal display device 10 having the vertical orientation, while the tilted liquid crystal molecules are twisted up to 180°, the orientation of the liquid crystal molecules at one edge of the projection 13A or 13B always corresponds to a direction of the polarizer absorption axis P and the orientation at another edge of the projection 13A or 13B always corresponds to a direction of the analyzer absorption axis A. When such the orientation of the liquid crystal molecules occurs, two dark lines shown in FIG. 2 appear along both edges of the projection 13A or 13B. The two dark lines deteriorate transmittance of a liquid crystal panel and then also, the contrast of liquid crystal display device 10 is deteriorated.
In addition, in the liquid crystal display device 10 shown in FIG. 1A and FIG. 1B, a tilt direction is controlled in the vicinities of the projections 13A and 13B but the tilt direction is not controlled in regions other than the vicinities. As a result, when a state of the liquid crystal display device 10 transits from the non-driving state in FIG. 1A to the driving state in FIG. 1B, the liquid crystal molecules start to be tilted in a given direction at the vicinities of the projections 13A and 13B and then tilt of the liquid crystal molecules at the vicinities propagate the liquid crystal molecules in the regions other than the vicinities. Finally, all liquid crystal molecules are tilted in the given direction. However, the response speed of such tilt propagation takes time. Thus, it is desired to improve the response speed. Especially, when a half tone is displayed by the tilt propagation, the tilt direction of the liquid crystal molecules located far from the projection patterns 13A and 13B is not defined since an electric field applied to the liquid crystal molecules is weak. Consequently, the response speed tends to delay.
Also, in the conventional liquid crystal display device 10 shown in FIG. 1A and FIG. 1B, the projection patterns 13A and 13B are required to be at least 1.2 μm height. However, when the projection patterns having a 1.2 μm height are formed by resist, a retardation of the liquid crystal layer 12 is decreased at the projection patterns 13A and 13B. Also, the decrease of the retardation degrades transmittance of the liquid crystal layer 12.